More Tube trains were withdrawn from service on London Underground today to allow safety checks, causing disruption for thousands of travellers.
Transport for London said around 20 District line trains were being checked after engineers discovered brackets which seemed to be cracked during a maintenance inspection earlier this week.
A TfL spokesman said: "Safety is our top priority and all District line trains are undergoing testing As a consequence it has been necessary to keep around 20 trains from service this morning to enable us to carry out these checks.
"We apologise to passengers for the disruption this is causing, but passenger safety is our top priority and this demonstrates that our safety regime works. We are working flat out to return the full fleet to service, but will only do so once all trains have been thoroughly examined."
TfL said the issue is not connected to the current dispute over job cuts with the Rail Maritime and Transport union (RMT) and the Transport Salaried Staffs Association (TSSA).
RMT leader Bob Crow said: "London Underground are now lurching from crisis to crisis as the impact of their cuts unleashes total chaos on the Tube network. Short-term savings measures have slashed maintenance schedules and ripped up the safety rule book and the consequences are now being rammed home on the District line and beyond.
"Mayor Boris Johnson is now presiding over a Tube system in meltdown, with lives at risk in a daily commuter lottery, and the time has come for him to stop the posturing, halt the cuts and meet with the Tube unions to work out a solution to this crisis."
Exploratory talks aimed at averting further strikes by Tube workers over the job cuts will resume today, sources told the Press Association.